Final answer:
The M checkpoint ensures sister chromatids are correctly attached to spindle microtubules during metaphase in mitosis. If this checkpoint fails, nondisjunction can occur, resulting in cells with improper chromosome numbers.
Step-by-step explanation:
The M checkpoint occurs near the end of the metaphase stage of mitosis and is crucial for ensuring that sister chromatids are correctly attached to spindle microtubules. This checkpoint, also known as the spindle checkpoint, verifies that the kinetochores of each chromatid pair are secured to spindle fibers from opposite poles of the cell, preventing anaphase from proceeding until this condition is met.
If the M checkpoint fails, improper separation of sister chromatids may occur, leading to nondisjunction of chromosomes. This means that daughter cells could end with an incorrect number of chromosomes, potentially causing genetic disorders or cell death.